Developer console messes up letterboxing size [1963869]

edit: upstream: https://bugzilla.mozilla.org/show_bug.cgi?id=1963869

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In 13.0a2, the developer console messes up with the browser size.

Steps to reproduce:

1. Visit some website in a tab (any site, as long as it isn't exempt from RFP)
2. Open another tab, and visit another website (same, the only requirement is that it isn't RFP-exempt)
3. Open the inspector, and make sure it's anchored to the page (e.g., to the right side of the browser)
4. Switch to the tab opened in 1.: it will have the same width of the tab opened in 2., and will be centered, even though the inspector isn't open in that tab.

/cc @ma1